Veraison is nearly done now. Most of the grapes have changed color and are now in their last phase of ripening until harvest. Sugar levels are shooting up as soon as the color change completes. From 5 Brix -measured in the green berries- to 15 Brix (Brix is a measure of % sugar content). We expect this to increase to around 25 Brix at harvest time. The taste of the grapes is already much more tolerable, but still too "green" with too high acidity. This will improve in the next month.Another visible change in the vines is that his year's new shoots turn brown and becoming woody.The picture shows this pretty nicely.

This series of postings describes how to make sensors and a sensor reader to measure soil moisture and help manage irrigation of a vineyard. Here is the second posting of the Vinduino project, describing how to make and Install gypsum soil moisture sensors.

A gypsum block sensor is a very simple and low cost device. Basically it consists of two electrically separated electrodes cast in a block of gypsum (Plaster of Paris). You can buy them on-line or make them yourself using the guidelines below. Because of their low cost, you can install many as you like in the vineyard.The electrical resistance between the electrodes is a measure of soil moisture level, or better: the soil water potential. The gypsum acts as a buffer for saline solutions making the measured electrical resistance of the sensor less responsive to salt in the soil.

Our home made gypsum sensor shows a resistance (blue line, the vertical axis is resistance in Ohms) from a few hundred Ω for saturated soils, to up to 80 kΩ for bone dry soil. Horizontal axis shows 15 minute measurement increments, when the sensor is drying in air.

Gypsum sensor have some less ideal properties, listed here for completeness:- Limited life, the gypsum decomposes and the sensor last only a few seasons- Hysteresis, the sensor quickly responds to a moisture increase, but is slow to reach equilibrium with the surrounding soil under drying conditions- Temperature dependency. The resistance varies over temperature, however this effect is not as strong as resistance variation over soil moisture.- Resistance can only be measured using AC voltage. There have been efforts to measure DC resistance with multi-meters, but this fails because DC current causes electrochemical effects in the sensor. The description of a simple to build sensor reader will follow in a later posting.The pictures below show the simple setup: use stainless steel machine screws (use #6-32 x 2") to prevent iron oxide penetrating the gypsum. Adding a piece of plastic window screen mesh helps improve the structural integrity of the sensor, extending its useful life. The electrodes are fixed before pouring the Plaster of Paris by plastic spreaders. The dimensions of the sensor are not critical, 1” diameter cylindrical by 2” length, but using our example dimensions will help get comparable resistance results. Isolate the terminals with liquid tape of something alike to avoid an electical path outside the sensor after connection of wires to the sensor terminals.

Quick coarse calibration can be done by soaking the sensor in distilled water, and let it air dry while measuring the sensor weight with an accurate scale and noting the resistance values over time.In practice, soil moisture is good when measuring below 1 kΩ. Values well above 1 kΩ tell you it’s time to open the drip irrigation valves.

Installation of the sensors is straightforward. Watermark - a brand of professional sensors- recommends measurements at different depths, depending of the type of crop and their typical root depths. Because of the auger length we had available, we only installed our sensors in pairs at 30” depth. The sensors need to be soaked in water before installation. To ensure good ground contact, have some “mud” tightly surround the sensor. Measurements can start 2 weeks after installation.

The deeper the sensors are installed, the longer the response time after irrigation start. At 30" depth, there is no measurable response after one full night of irrigation. Therefore we now measure once a week.

Wildlife destroyed our electric sensor cables within one week (!!) after installation. We now have PVC tubes to protect the wires. So far that seems effective.

Today we found the first cluster of grapes starting to change color. Veraison has started in Temecula, two weeks sooner than last year.

Veraison represents the transition from berry growth to berry ripening. The young green berries are high in acidity, and hard to the touch. When the color of the berry changes, multiple changes start taking place. The skin becomes thinner, the color turns to purple, and the composition of the acids changes. Also, important for the alcohol level of the future wine, the berry sugar content increases. We're about two months from harvest now. Time to prepare for crush season.

Speaking of that. We ordered a new crusher/destemmer to process the grapes faster once they come in from the vineyard. This will help getting the maximum goodness from our grapes into the wine.

In Southern California we need irrigation to get good yield from our grape vines, or even to protect them from withering away. In some parts of our vineyard the vines thrive, and grow vigorously. In some other spots, mainly the back side of the hill, the vines stay behind and sometimes even struggle.

Last year we used a calculation method, based on evapotranspiration data from our local weather station, to estimate the weekly water requirement per vine. However, that method did not address the differences we observed in vine development per location. We came up with a new approach to keeping track of our irrigation this vintage. We installed gypsum soil moisture sensors at the trouble spots, and also installed sensors at a "good" spot, sensor location 1, as a reference. We started weekly measurements in May, and now in July, see clear indications that the soil moisture is decreasing, so we need to step up the irrigation. Meanwhile, having better visibility on irrigation water quantity needed, the vineyard looks much better than same time last year.

To measure the electrical resistance - an indication of soil moisture level- of a gypsum sensor reliably, you need a special device that measures the resistance of the sensor with an AC voltage. Using a standard multi-meter does not work, as this device uses a DC voltage and electrochemical effects in the sensor will interfere with the moisture related resistance. Having a background in electronics engineering, the idea came up to build a sensor reader ourselves. This how the Vinduino project got started. In next blog posts there will be a more detailed descriptions of how to make gypsum sensors and the gypsum sensor reader